The hell!
This NeXTdimension card actually works! Apparently my main error after all this RAM swapping was to enable parity checking, since main RAM was now parity-enabled (but SIMMs in the NeXTdimension are not). Also moved the four modules towards the edge of the dimension board.
I have no idea why we didn't get it up and running back in the 1990s and we always assumed the card was faulty.
System log sais:
NeXTdimension Board in Slot 2:
ROM Version 43, Memory Controller step 1, i860 step C.1
32 MBytes of DRAM installed, PAL video configured
NeXTdimension server running
(Attached photo is slightly weird, the phone cam could not be convinced to use a sane brightness, and gamma correction messed it up.)